As others have said, the majority of older razors 10-32, and most newer handles are M5x0.8.
If you were to tap the handle at M5x0.8, you will have something that will work with most heads, Just not the Fatip. The fit may not be as snug and precision feeling as say a Paradigm or Raw razor.
A head threaded at 10-32 will fit with most handles 10-32 and M5x0.8. It might just not be the precision fit of the high end razors if using the M5 handle.
The Fatip is actually smaller, not larger as said earlier. I believe I have read it is M4.5x.75 but don't quote me on that. I just know it is smaller.
Tough to find something that works for all of them. The closest is the Weber Bulldog or older iKon Bulldog polished (which I think are the same handle other than a logo). They are barely small enough for the Fatip, but large enough for most modern heads. I doubt they would be large enough for a precision M5 head though. I will try and check with a Paradigm tonight. Not sure they are M5, but it feels big and precise.
If I were going to try and make handles for sale, I would be looking for a niche of jist being small enough to fit the Fatip. Fatip owners are hungry for nice handles, moreso than Paradigm users are hungry for nice handles, and if done like the Weber, it would fit most moderns. . ..but I don't know what size this is, or if they went with something custom to try and be universal. If your friend is just making one for you, I would go with the M5 (unless you are a Fatip fan).
For the sake of completeness, I know that GEM and Everready heads have female threading. 10-32 on the newr ones, but the older ones (like 1910's and 1920's) have the odd 8-24 or some weird similar size.
